A radio receiver selects a desired signal amongst several potentially strong interfering signals. To operate properly, it must minimize circuit effects that add unwanted artifacts to the received signal such as distortion, noise, and dc offsets. In practice, it's possible for these artifacts to degrade performance and even saturate the receiver.
A typical radio receiver is shown in FIG. 1. It uses a single RF downconverter to translate the received signal to baseband. In practice, the baseband frequency equals dc (for direct conversion architectures) or lies near dc (low-IF approach). At baseband, the A/D converters sample the received signal and convert it into digital data. An automatic gain control (AGC) system in the digital MODEM allows the receiver to process a wide variety of signal levels.
The direct conversion and low-IF receiver architectures allow highly integrated radio solutions. Unfortunately, these approaches also suffer from a number of problems. Since direct conversion schemes center the baseband signal at dc, these receivers are sensitive to dc offsets and even order distortion. Narrowband signals like GSM/EDGE present a greater challenge.
Traditionally, the radio receiver includes multiple feedback loops to remove dc offsets at critical points and to avoid saturation of its high-gain stages. The feedback loops affect the system's response and must be stable. Moreover, the resulting response cannot affect the desired signal. This can be challenging since the receiver's gain changes dynamically.
In practice, the dc offset correction loop generally uses analog signal processing techniques. Unfortunately, the analog circuits show sensitivities to signal levels, process parameters, supply voltages, and layout effects. As such, they oftentimes need to be redesigned when conditions change.
It therefore would be advantageous to eliminate as much analog signal processing as possible and rely on digital methods to remove the dc offsets.